Hi bacon, I'm studying Latin right now, but I went through Basics of Biblical Greek about a year and a half ago. Although I've forgotten alot of what I learned (I was seduced into Latin and put Greek on the back burner for now), I know that a relative pronoun only needs to agree in number and gender with its antecedent, and not case (in both Latin and Greek, or English for that matter). Take this sentence: "I was talking to my friend, who likes to study Greek, when my phone rang." 'who' is a relative pronoun referring back to 'my friend'; 'my friend' is the indirect object of the verb 'was talking', yet 'who' is the subject of the verb 'likes' in its clause (it would be 'whom' if it were the object). So the case of a relative pronoun is decided by its function within its clause.
Someone else can answer #2 since my Greek is way too rusty.
As far as the middle voice is concerned I wouldn't worry about it too much. If I remember correctly the middle voice was used alot more in say Attic Greek, but by Koine Greek was dropping out of usage. Plus, as Mounce says, most middles are deponents which just means that they are active verbs using passive forms.
I hope this helps some, hopefully someone with more knowledge and experience can give some input.